The present invention relates to “medical devices” as defined by the 14 June 1993 Directive 93/42/EEC of the Council of the European Communities, and more specifically to the “active implantable medical devices” as defined by the 20 June 1990 Directive 90/385/EEC of the Council of the European Communities, such as those implanted devices that continuously monitor a patient's cardiac rhythm and deliver if necessary to the heart electrical pulses for stimulation, cardiac resynchronization, cardioversion and/or defibrillation in case of a rhythm disorder detected by the device, as well as neurological devices, cochlear implants, devices for pH measurement, and devices for intracorporeal impedance measurement (such as the measure of the transpulmonary impedance or of the intracardiac impedance).
The present invention relates more particularly to devices that implement autonomous implanted capsules that are free of any physical connection to another medical device, which may be implanted (such as the can of a stimulation pulse generator) or not implanted (such as an external programmer or monitoring device for remote monitoring of the patient). Communication between such an autonomous implanted capsule and another medical device is typically conducted by the interstitial tissues of the body and is known as intracorporeal communication or human body communication (“HBC”).
These autonomous implanted capsules using HBC signals for communications are for this reason called “leadless capsules” to distinguish them from the electrodes or sensors placed at the distal end of a lead, the lead being traversed throughout its length by one or more conductors connecting by galvanic conduction the electrode or the sensor distal to a generator connected at the opposite, proximal end, of the lead. Such leadless capsules are for example described in U.S. 2007/0088397 A1 and WO Patent Publication No. 2007/047681 A2 (Nanostim, Inc.) or in U.S. Patent Publication No. 2006/0136004 A1 (EBR Systems, Inc.).
These leadless capsules (which for convenience also are referred to herein simply as “capsules”) can be epicardial capsules fixed to the outer wall of the heart, or endocardial capsules fixed to the inside wall of a ventricular or atrial cavity. Capsule attachment to the heart wall is usually obtained by a protruding anchoring helical screw, axially extending out of the body of the capsule and designed to penetrate the heart tissue by screwing to the implantation site.
This capsule typically includes detection/stimulation circuitry to collect depolarization potentials of the myocardium and/or to apply electrical pulses to the site where the capsule is located. In such case, the capsule includes an appropriate electrode, which can be included in an active part of the anchoring screw. It can also incorporate one or more sensors for locally measuring the value of a parameter such as the oxygen level in the blood, the endocardial cardiac pressure, the acceleration of the heart wall, the acceleration of the patient as an indicator of activity, etc.
It should be understood however that the present invention is not limited to a particular type of capsule, and is equally applicable to any type of leadless capsule, regardless of its functional purpose.
Of course, for the remote exchange of data, the leadless capsules incorporate transmitter/receiver circuitry for wireless communication. Several techniques have been proposed for wireless communication between the autonomous implanted capsules and a main remote device to centralize the information collected by the capsule and send to the capsule, if necessary, appropriate controls (particularly where the capsule may be an implanted pacemaker, defibrillator or resynchronizer, a subcutaneous defibrillator, or a long-term event recorder).
Thus, U.S. Patent Publication No. 2006/0136004 A1 proposes to transmit data by acoustic waves propagating inside the body. Although this technique is safe and effective, it nevertheless has the disadvantage of requiring a relatively high emitting power given the attenuation of acoustic waves into the body, and therefore allows only relatively low data transmission rates.
U.S. Pat. No. 5,411,535 proposes another communication technique, based on the use of radiofrequency waves (RF). Again, a relatively high transmission power is required, and the attenuation of these waves by intracorporeal tissue is a major barrier to their spread.
Another communication technique has been proposed by U.S. Pat. No. 4,987,897, but it is a data exchange with an external device (programmer), through the patient's skin rather than an intracorporeal transmission. This transmission is over a short distance, between, on the one hand, the housing of a pacemaker implanted in a subcutaneous pocket and, on the other hand, an external programmer placed against the skin near the generator. Currents therefore circulate through the skin in a very distant area from the sensitive areas, particularly in a very distant area from the myocardium, which avoids any risk of disruption of the natural or stimulated depolarization waves of the latter.
The U.S. Patent Publication No. 2007/0088397 A1 also proposes to use the stimulation pulses produced by a capsule as a vehicle for the transmission of data previously collected or created by the capsule. To this purpose, the pulse, instead of presenting a monotonic variation of voltage, is interrupted in a controlled manner for very short durations in order to create (modulate) in the profile of the pulse very narrow pulses whose sequence corresponds to binary encoding of information to be transmitted.
The processing of signals exchanged between the capsules and/or the other medical device (e.g., an implanted pacemaker or defibrillator) or, more generally, between the various independent devices implanted in the patient's body or not, implies to synchronize the respective processing circuits of these devices, or at least to determine their degree of desynchronization. If it is known, such a synchronization parameter may be included in the various signal processing calculations, or serve to apply an appropriate temporal correction to the timing of the received or transmitted signals. This “software synchronization” thus can be achieved which does not need to apply a corrective action or adjustment of the clock circuits of the various devices.
Further, to the extent that the various devices are all physically independent, each has its own clock that provides control of the various circuits, including the circuits for digital signal processing. These independent clocks have a frequency difference which, even if it is small, introduces over time a desynchronization of these devices that becomes necessary to identify and compensate for.
In a related area, U.S. Patent Publication No. 2009/030484 A1 proposes to synchronize the sequencing of two separate hearing aids, arranged on both ears of a patient, to reduce mutual interference that may be generated. These hearing aids are equipped with their own clocks, and with means for exchanging resynchronization data to readjust these two clocks.
The problem of synchronous operation of leadless implants is also discussed by US Patent Publication No. 2007/0255330 A1, which refers to measuring the time interval between the detection in two different regions of the patient of the same physiological event, or by US Patent Publication No. 2005/0197680 A1, which acts on a phase-locked loop for fine control of the clock frequency.
The frequency drift is increased by the fact that, to avoid excessive consumption, the implanted devices use clocks operating at a relatively low frequency (typically in the range of 32 kHz) and thus with a lower temporal resolution (about 30 μs) which does not allow a detailed evaluation of the synchronization timing differences. This is particularly mentioned, as such, by the US Patent Publication No. 2007/0088394 A1, but outside of the context of synchronization between the implants.
Indeed, whatever the technique used, the processing of the received or emitted HBC signal by a capsule requires considerable energy compared to the energy resources available to the capsule. Given its autonomous nature, the capsule can in fact use only its own resources such as an energy harvester circuit (by the movement of the capsule) and/or a small integrated battery, which energy sources are in themselves well known to persons of ordinary skill in the art and form no part of the present invention, and therefore are not described in detail.
The management of the available energy is, however, a crucial point in the implementation of HBC signals to and from autonomous implanted capsules, and it is essential to develop techniques that minimize the energy requirements of these capsules.